The present invention relates generally to a thermal barrier coating system for a component that is exposed to high temperatures, such as a gas turbine engine component (e.g., blades, vanes, etc.). More particularly, the present invention relates to a coating including a low aluminum content, where the coating is suitable for use as a bondcoat in a thermal barrier coating system.
A gas turbine engine component (“component”), such as a blade tip, blade trailing edge, blade platform, vane trailing edge, or vane platform, is typically exposed to a high temperature and high stress environment. The high temperature environment may be especially problematic with a superalloy component. Namely, the high temperatures may cause the superalloy to oxidize, which then decreases the life of the component. In order to extend the life of the component, a thermal barrier coating system (TBC system) may be applied to the entire superalloy component or selective surfaces, such as surfaces of the superalloy component that are exposed to the high temperatures and other harsh operating conditions. A TBC system reduces the temperature of the underlying material (also generally called the “substrate”) and helps inhibit oxidation, corrosion, erosion, and other environmental damage to the substrate. Desirable properties of a TBC system include low thermal conductivity and strong adherence to the underlying substrate.
The TBC system typically includes a metallic bondcoat and a ceramic topcoat (i.e., a thermal barrier coating or TBC topcoat). The bondcoat is applied to the substrate and aids the growth of a thermally grown oxide (TGO) layer, which is typically aluminum oxide (Al2O3 or “alumina”). Specifically, prior to or during deposition of the TBC topcoat on the bondcoat, the exposed surface of the bondcoat can be oxidized to form the alumina TGO layer. The TGO forms a strong bond to both the topcoat and the bondcoat, and as a result, the TGO layer helps the TBC topcoat adhere to the bondcoat. The bond between the TGO and the topcoat is typically stronger than the bond that would form directly between the TBC topcoat and the bondcoat. The TGO also acts as an oxidation resistant layer, or an “oxidation barrier”, to help protect the underlying substrate from damage due to oxidation.
During use in a gas turbine engine, the TGO thickens as aluminum diffuses into the TGO from the bondcoat and oxygen diffuses into the TGO from the combustion products and cooling air in the turbine gas path, reacting to form more alumina TGO. As the TGO thickness increases, it carries a proportionally larger share of any stresses that arise in the TBC system. Eventually, this share of the stress exceeds the strength of the TGO, leading to its failure. Once the TGO fails, the TBC topcoat spalls from the bondcoat because there is little to no TGO to provide adhesion.
Most bondcoats are designed to maintain slow growth of the TGO and to insure that the TGO consists of pure alumina. If the amount of aluminum that diffuses from the bondcoat to the TGO is insufficient to sustain growth of pure alumina, spinel oxide may form. Spinels grow quickly because they do not act as oxidation barriers. A TGO containing spinels has a significantly shorter life than a pure alumina TGO because spinels are not as strong as pure alumina.
The TBC topcoat may consist of a zirconia material that includes yttria as a stabilizing material. The primary role of the TBC topcoat is to provide insulation, thereby reducing the temperature of the bondcoat and the substrate. Thus, TBC ceramic topcoats are designed to have low thermal conductivity. There are various techniques of applying the TBC topcoat on the component, including air plasma spraying, vapor deposition and thermal spraying methods such as a high velocity oxy-fuel method.
The bondcoat is important to the life of the TBC system. If the bondcoat fails, the TBC topcoat may spall, after which the TBC system quickly deteriorates. TBC topcoat spallation exposes the bondcoat to the high-temperature, oxidizing environment of the turbine gas path, as well as to any corrosive species that may be in the gas path arising from impurities in the fuel and ingested fine particulates or contaminants. The higher temperatures and any deposits of contaminants accelerate the oxidation or corrosion of the bondcoat, eventually consuming the bondcoat. Once the bondcoat is consumed, the harsh environment attacks the underlying substrate. A deteriorated TBC system is undesirable because it may shorten the life of the component, and at the very least, requires the component to be taken out of service in order for the TBC system to be repaired. Whenever possible, the TBC system is removed and replaced prior to its complete failure to ensure no degradation of the underlying substrate.
In addition to its role in protecting the substrate from oxidation and corrosion, it is preferable that the bondcoat adheres well to the substrate with minimal reaction and interdiffusion.